The immune system is subject to multiple layers of regulation that promote tolerance to self and that are critical for the prevention of autoimmune diseases, such as Type 1 diabetes. A key player in the maintenance of immune tolerance is the Autoimmune Regulator (Aire) gene. Aire was identified as the defective gene in the human autoimmune syndrome Autoimmune Polyglandular Syndrome Type 1, highlighting the critical importance of this gene in tolerance. Aire acts within specialized medullary thymic epithelial cells (mTECs) to promote the expression of hundreds of self-antigens for the purpose of removing developing self-reactive T cells, a process known as negative selection. Recently, we have described an additional site of Aire action within a unique population of antigen-presenting cells found in peripheral lymphoid organs that we have termed extra-thymic Aire-expressing cells (eTACs). As in mTECs, Aire acts in eTACs to promote the expression of many self-antigens that are distinct from those controlled by Aire in the thymus, suggesting a complementary role of eTACs in promoting tolerance in the periphery. Indeed, eTACs are capable of presenting self-antigens to peripheral T cells to cause deletion or functional inactivation of the self-reactive cells, thus preventing autoimmunity. Much remains unknown about this novel cell type; however, the discovery of the bone-marrow origins of eTACs opens up the potential for expansion and manipulation of this cell type for therapeutic applications in promoting self-tolerance. We have developed a powerful set of genetic tools that will allow us to define the growth and development of these cells as well as identify potentially unique pathways of peripheral tolerance induction, through comparisons with other types of antigen-presenting cells. We hypothesize that eTACs represent a unique tolerogenic population of APCs with an important role in mediating peripheral tolerance and can serve as a novel therapeutic target for tolerance induction. Therefore, our specific aims are: (1) to define and characterize the lineage and growth requirements of eTACs, (2) to define the sites and mechanisms of eTAC action, and (3) to determine how loss and gain of function in eTACs can modulate disease in models of T1D. Taken together, our studies will help determine origins of eTACs, the mechanisms by which they enforce tolerance, and how we may employ them in the prevention of Type 1 diabetes and other autoimmune diseases.